The present invention relates to a cruise control apparatus for a vehicle for automatically maintaining the speed of the vehicle at a target level.
There has hitherto been known a cruise control apparatus for a vehicle in which the opening degree of a throttle valve disposed in an intake manifold of an internal combustion engine is controlled by an actuator on the basis of the speed of the vehicle as sensed by a speed sensor and a target speed. A cruise controller calculates a control quantity for the actuator whereby the actuator is driven to adjust the opening degree of the throttle valve to make the vehicle speed equal to the target speed. Such an actuator comprises a motorized actuator having a DC motor as its drive source which is able to move the throttle valve to its fully open and closed positions. With the use of such a motorized actuator, the gear ratio of an automatic transmission is changed automatically on the basis of the relation between the vehicle speed and the opening degree of the throttle valve such as when the vehicle ascends a steep slope. In this case, in the event that a gear shift is effected with the throttle valve being at a relatively large degree of opening, a great change in the engine output torque will take place, giving rise to a substantial shock to the vehicle. This causes an unpleasant sensation to the driver and passengers in the vehicle. In extreme cases, the vehicle can be subject to a sudden acceleration, thus arousing a fear of runaway of the vehicle in the driver and passengers. To cope with such a situation, the control output of the actuator is limited such as by the output signal of a potentiometer which senses the opening degree of the throttle valve. This will be described below in more detail with reference to FIG. 5.
FIG. 5 illustrates an typical example of this type of conventional cruise control apparatus for a vehicle which is disclosed in Japanese KoKai No. 58-39311. The apparatus illustrated includes a plurality of cruise control switches such as a set switch 101 for setting in or starting cruise control, and a cancellation switch 102 which is automatically operated upon application of the brakes of the vehicle to cancel cruise control. A speed sensor 103 for sensing the speed of the vehicle comprises a rotary member 103a having four magnetic poles disposed at equal circumferential intervals with respect to each other and being connected to an unillustrated transmission through an unillustrated cable for synchronized rotation with an output shaft of the transmission, and a reed switch 103b disposed adjacent the rotary member 103a and having a pair of contacts one of which is connected to ground and the other of which is connected to a control unit 106. The reed switch 103b of the speed sensor 103 is magnetically switched on or off every time each one of the four magnetic poles of the rotary member 103a passes near its contacts of the reed switch 103b, generating a pulsating signal comprising a train of pulses having a frequency proportional to the speed of the vehicle. The control unit 106 including a calculator 106a in the form of a microcomputer is connected through a main or power switch 105 to a power supply 104 in the form of a battery mounted on the vehicle. The control unit 106 receives the output signals from the set switch 101, the cancellation switch 102 and the speed sensor 103, and it operates to perform various calculations and processing for automatic cruise control whereby the speed Vs of the vehicle is made to match the target speed Vr. Based on these calculations and processing, the control unit 106 generates various control outputs for doing such automatic control which is fed to a throttle actuator 107 in the form of a motorized actuator. The throttle actuator 107 is operatively connected, through a link mechanism comprising a link 107a in the form of a rotary disk and a wire 107b, with a throttle valve 109 which is disposed in an intake manifold 108 of the internal combustion engine for controlling the amount or flow rate of intake air sucked into the engine through the intake manifold 108. The throttle valve 109 is also operatively connected with an unillustrated accelerator pedal through the link mechanism. The link or disk 107a rotates around its axis in correspondence with the rotational motion of the throttle valve 109 so that the rotational angle of the disk 107a corresponding to the opening degree of the throttle valve 109 is sensed by an unillustrated potentiometer incorporated in the actuator 107 and it is sent to the control unit 106 as a throttle signal. The disk-shaped link 107a is connected with the actuator motor through an unillustrated electromagnetic clutch which is controlled by the control unit 106.
The operation of the above-mentioned conventional cruise control apparatus will now be described in detail below. First, when the driver turns on the main or power switch 105, electric power is supplied from the battery 104 to the control unit 106 which is thus energized to process the output signal from the speed sensor 103. During the time the vehicle is travelling, the speed sensor 103 generates a pulsating signal containing a train of pulses having a frequency proportional to the vehicle speed Vs. The periods of successive pulses from the speed sensor 103 are measured by the control unit 106 to calculate the vehicle speed Vs. When the driver switches the set switch 101 on, a set signal is supplied to the control unit 106 which is thereby operated to start cruise control so that the vehicle speed Vs at that time is stored therein as a target speed Vr.
Thereafter, the control unit 106 compares the vehicle speed Vs successively sensed by the speed sensor 103 with the target speed Vr and generates a control signal to the throttle actuator 107 which is thereby driven to control the opening degree of the throttle valve 109 in such a manner as to match the vehicle speed Vs with the target speed Vr. More specifically, in cases where the actual vehicle speed Vs is less than the target speed Vr, the control unit 106 generates a throttle opening signal for driving the throttle valve 109 in an opening direction by an appropriate amount. On the contrary, if the actual vehicle speed Vs is greater than the target speed Vr, the control unit 106 generates a throttle closing signal for moving the throttle valve 109 in a closing direction by an appropriate amount. In this manner, the vehicle is controlled to travel at the constant target speed Vr without requiring any driver's accelerator pedal operation. On the other hand, if the driver applies the brakes during cruise control, the cancellation switch 102 is automatically operated to generate a cancellation signal to the control unit 106 which then immediately outputs a clutch release signal to the throttle actuator 107 to release or disengage the built-in electromagnetic clutch. As a result, the driver is able to freely control the throttle valve 109 through the accelerator pedal at his or her own will, so that the speed of the vehicle is accordingly controlled manually by the driver.
As can be seen from the foregoing description, with the conventional cruise control apparatus, for the purpose of driving the actuator 107 while avoiding an automatic gear shift by the automatic transmission apparatus on the basis of the relation between the vehicle speed and the opening degree of the throttle valve 109 during cruise control, the actuator 107 is required to have a built-in potentiometer or the like for sensing the throttle opening, or it is necessary to sense the output signal from a throttle sensor which senses the opening degree of the throttle valve 109. In addition, the provision of electrical elements such as a potentiometer or the like built in the actuator 107 needs electrical circuits incorporated in the actuator 107, thus resulting in a complicated construction and high costs of manufacture. Moreover, the actuator 107 is generally accomodated in an engine compartment of a vehicle. In this case, in order to protect the electrical elements and circuits incorporated in the actuator 107 against intrusion of moisture, rain water, external dust and the like, special considerations or measures should be taken into account. Further, the potentiometer for sensing the throttle opening generally transmits to the control unit 106 its output signal in the form of an analog signal, which is required to be converted into digital form suitable for processing by the microcomputer 106a in the control unit 106. This results in a complicated circuit design or arrangement.